1. Field of the Invention
This invention relates to a novel process for making ultrahigh molecular weight linear polyethylene (UHMWLPE). This novel UHMWLPE, in the form of a shaped article, exhibits a unique combination of properties making the material useful as a bearing surface, in general, but particularly useful as a prosthetic hip joint cup and as other prosthetic shapes for replacement of other joints of the human body. This article is the subject of copending U.S. Ser. No. 07/426,916 filed on Oct. 24, 1989 which is a continuation-in-part of co-pending U.S. Ser. No. 07/288,576 filed Dec. 22, 1988, which in turn is a continuation-in-part of co-pending U.S. Ser. No. 07/278,913, filed Dec. 2, 1988.
2. Description of the Prior Art
In U.S. Pat. No. 3,944,536 (March 1976), Lupton et al describe UHMWPE in the form of a fabricated article exhibiting an elastic modulus of 340,000 to 500,000 psi, a tensile impact strength of 140 to 600 ft lb/in.sup.2, a density of 0.95 to 0.98 g/cc at 25.degree. C., a crystalline melting point of 142 to 148.degree. C. (as measured by differential thermal analysis) and a unique crystalline form characterized by the absence of fold spacings of 50-2000 Angstrom units (.ANG.) and the presence of crystal spacings of about 10,000 .ANG.. The critical feature of the process of producing this UHMWPE is disclosed to involve inducing crystallization of the molten polymer above 150.degree. C. by rapidly increasing the applied pressure from an initial level of 1 to 1000 atmospheres to a second level of 2000 to 7000 atmospheres and then cooling rapidly while maintaining a pressure sufficient to maintain the polyethylene in the solid phase until the temperature is below the crystalline melting point of the polyethylene at atmospheric pressure.
In Kunstuffe German Plastics 77 (1987) pp. 617-622, in an article entitled "Ultrahigh Molecular Polyethylene for Replacement Joints", Eyrer et al. point out that the service life of joint replacements made of UHMWPE is limited. Analysis of the damage to over 250 explanted hip cups and tibial plateaus revealed a changed property profile which they explained by post-crystallization resulting from oxidative chain decomposition. They suggested optimizing the processing of polyethylene under higher pressure and higher temperature to increase the degree of crystallinity. The Eyrer et al. product displays a creep of above 5% at a compression of 1000 psi (6.9 N/mm.sup.2) for 24 hours at 37.degree. C.
One of the most remarkable advances in the medical field in recent years is the development of prosthetic joints, particularly the load bearing hip. The crippled and sometimes bed ridden elderly can walk again. The key to this development is UHMWPE because, not only does it have the necessary impact strength, but it initiates no adverse blood reactions. But at present, these prosthetic joints are limited to the older, less active segment of the population because the polymer tends to creep under the pressure that a younger more active person might develop while involved in recreation or employment. The creep would cause the loss of the desired tolerance required between the plastic socket and the polished metal ball attached to the femur. These changes in dimensions disturb the distribution of walking forces which in turn accelerates more creep and wear. Eventually the increased pain requires a traumatic revision operation. One objective of this invention is to provide a process for making UHMWPE prosthetic joints with improved creep resistance hence removing some of the age restriction existing with regard to the present polyethylene joints.